The present invention relates to a porous gas diffusion electrode for membrane fuel cells on an ion-conducting polymer membrane as well as to a method of its production. The use of gas diffusion electrodes for fuel cells has long been state of the art. Several methods for producing these electrodes have been developed for the membrane fuel cell using electrocatalysts based on platinum catalysts or platinum alloy catalysts on conductive carbon carriers.
The control of the contact of the three phases catalyst/electrolyte/gas has proven to be particularly difficult in a solid electrolyte system such as is constituted by a membrane fuel cell with an ion-conducting membrane as electrolyte. Traditional gas diffusion electrodes for use in acidic fuel cells (e.g. the phosphoric-acid fuel cell) are generally produced from a mixture of polytetrafluoroethylene (PTFE) and an electrocatalyst of platinized black which mixture is absorbed on a gas distributor structure. After a tempering procedure this yields a porous, surface-rich and partially hydrophilic, partially hydrophobic structure of the electrode. This enables, during operation in a fuel cell with a liquid electrolyte, good access of the working gases to the electrochemically active centers with simultaneous good wetting by the electrolyte. The entrance of the liquid electrolyte into the depth of the electrode opens up a sufficiently high number of these electrochemically active centers.
A membrane fuel cell consists of, in its essential components, a membrane of an ion-conducting polymer, also designated herein as "ionomer", with gas diffusion electrodes applied on both of its sides as cathode and anode of the fuel cell. The membrane has two large surface areas, or sides and a relatively small thickness. The cathode and anode electrode contain suitable fine electrocatalysts for accelerating the oxidation of the fuel, as a rule hydrogen, on the anode and reduction of the oxygen on the cathode. The polymer membrane forms the electrolyte. The conduction of current through the membrane takes place by the transport of protons.
Platinum is preferably used as the catalytically active component of the electrocatalyst, which platinum can also be alloyed with one or several metals of the groups VB, VIB, VIII and IB of the Periodic Table of Elements. The optimum particle size of the catalytically active alloy particles is in a range of 2 to 10 nm. For use in the electrodes of fuel cells the catalytically active components are employed as carrier catalysts, that is, the metal or alloy particles are deposited on a carrier which can be finely divided, electrically conductive carbon materials such as e.g. furnace black and worked in this form into the electrodes. Alternatively, however, there is also the possibility of bringing the alloy particles directly into the electrode material without carrier.
The access of the electrolyte into the depths of the electrode is not readily possible in a membrane fuel cell. The so-called three-phase zone remains, without special provisions, limited to the range of the contact surfaces between membrane and electrodes.
U.S. Pat. No. 4,876,115 teaches a method of modifying commercial gas diffusion electrodes which are usually used in liquid electrolyte systems. These electrodes contain as binder hydrophobic particles of polymeric PTFE, which simultaneously regulates the wetting properties of the electrode and stabilizes the porosity of the electrode layer. In order to improve the contact of the three phases; i.e. catalyst/electrolyte/gas, the prefabricated electrode is impregnated with a solution of a proton-conducting material by spraying and then bringing into contact the sprayed side with the proton-conducting membrane. The porosity of the electrode is formed only by the intermediary spaces between the particles of the electrocatalyst and of the hydrophobic PTFE.
If the electrode is impregnated with an ionomer as proton-conducting material only approximately 10 .mu.m of the depth of the electrode is reached by the electrolyte as a result of this pretreatment. Therefore, a large part of the electrocatalyst remains electrochemically unused in the electrode, which is generally 100 to 200 .mu.m thick. Performance data similar to that obtainable with conventional electrodes with surface concentrations of 4 mg Pt/cm.sup.2 can be achieved with these electrodes at surface concentrations of 0.35 to 0.5 mg Pt/cm.sup.2. However, the maximum concentration when using carrier catalysts is limited to values of approximately 0.5 mg Pt/cm.sup.2 on account of the electrochemical utilizable layer thickness, which is only about 10 .mu.m thick. A performance increase by raising the surface concentration of catalyst is therefore possible only to a slight extent. This excludes those electrodes from applications which, according to the current state of knowledge, require catalyst concentrations of 4 mg Pt/cm.sup.2 and more, as is necessary e.g. in a direct methanol fuel cell. In order to produce such electrodes with higher, electrochemically utilizable concentrations, carrier-free catalysts must be used.
The production of electrodes for membrane fuel cells by spraying a dispersion of dissolved ionomer, electrocatalyst and PTFE onto the heated membrane is described by S. Escribano et al. (Editions de l'Etcole Polytechnique de Montreal 1995, pp. 135-143). The electrodes are only a few micrometers thick and have pores with pore radii of approximately 50 nm. Thereafter, gas distributor structures are hot pressed onto the electrode layers. Temperatures near the melting point of PTFE (320.degree.-360.degree. C.) are used in that process in order to sinter the PTFE particles. PTFE functions in these electrodes as binder and hydrophobing agent.
According to U.S. Pat. No. 5,211,984 PTFE is eliminated as binder and hydrophobing agent and a non-self-supporting electrode is obtained consisting only of catalyst and ionomer. To this end a suspension of dissolved ionomer and platinized black is applied in a method variant onto a PTFE carrier, dried and the pre-formed electrode is pressed with the PTFE carrier onto a membrane. The PTFE carrier can subsequently be drawn off free of residue. The electrode with a thickness of approximately 10 .mu.m adheres very well to the membrane after the hot pressing procedure.
The electrode produced in the manner shown in the prior art consists of a dense layer of ionomer and electrocatalyst. The electrode layer therefore contains essentially no pores and also no hydrophobic additives. The electrode layer is therefore limited to a maximum thickness of 10 .mu.m. This maximum layer thickness also assures a sufficiently good transport of the oxygen to the catalyst particles by diffusion through the ionomer. A layer thickness of less than 5 .mu.m is preferably striven for. Even these electrodes achieve performance data with surface concentrations of less than 0.35 mg Pt/cm.sup.2 similar to that of conventional electrodes with concentrations of 4 mg Pt/cm.sup.2. However, performance increases can hardly be achieved with these electrodes either when using carrier catalysts since the surface concentration can not be raised on account of the low layer thickness.
Ionomeric polymer membranes can be present in an acidic, proton-conducting H.sup.+ form or, after exchange of the protons by monovalent ions such as e.g. Na.sup.+ and K.sup.+, in a non-acidic Na.sup.+ or K.sup.+ form. The non-acidic form of the polymer membranes is usually more resistant to temperature stresses than its acidic form. The membranes are therefore preferably used in their Na.sup.+ form for the applying of the electrode layers--likewise the ionomer present in solution for the electrode layer. In the last method step of the production of electrodes the polymeric material is transformed by so-called reverse protonation back into the acidic, proton-conducting form. This usually takes place by treating the unit consisting of electrode/membrane/electrode (EME-unit) in sulfuric acid.
According to U.S. Pat. No. 5,211,984 the robustness of the electrode layers can be further improved if the dissolved ionomer is present in a thermoplastic form in the suspension of catalyst and ionomer solution used to produce the electrode layer. The thermoplastic form is obtained by ion exchange of the proton-conducting form of the ionomer with e.g. tetrabutylammonium cations.
U.S. Pat. No. 4,469,579 describes the production of porous electrodes on solid electrolyte membranes for use in sodium chloride electrolysis cells. The electrodes are produced by spraying the membrane with a dispersion of an electrocatalyst in a solution of an ionomer, which dispersion can contain pore-forming materials in order to produce pores for the transport of the gases formed during the electrolysis. The pore-forming materials are dissolved out of the electrodes after removal of the solvent, that is, after drying of the electrodes.
Oxides, hydroxides, nitrates or carbides of various elements with particle sizes between 0.025 mm and 3 mm have been suggested in the prior art as pore-forming materials. Fibrous pore-forming materials with lengths up to 50 mm are preferably used. A preferred pore-forming material is zinc oxide, which is dissolved out of the electrode with sodium hydroxide solution after drying.
The German publication 15 46 701 corresponding to U.S. Pat. No. 3,385,780 discloses a method for the preparation of a porous electrode for fuel cells. According to the method described therein, a finely divided electrocatalyst is mixed with a hydrophobic polymer and a filler for the preparation of a first mixture. A second mixture is than prepared without the electrocatalyst. Both mixtures are than conveyed to a press in two separated sheets and are pressed at a pressure of 350 and 840 bar. Subsequently, the filler is removed so that a unitary porous body is obtained. The fillers that can be used include a thermally heat decomposable material or a material which can be leached out and removed with a strong base. Suitable fillers include ammonium oxalate, ammonium carbonate, silica gel, alumina and calcium carbonate. As hydrophobic polymers, there is mentioned polytetrafluoroethylene as an example. No disclosure is found in this publication of the porosities obtained in the resulting electrodes. Because of the high pressures used, it may be presumed that the electrode material is highly compacted and that the porosity is determined based only on the above mentioned fillers which are used and subsequently are removed in making the electrodes.
European 0 622 861A1 discloses a membrane electrode structure. In order to prepare the electrode, a so called ink is prepared which is formed of a 5% ionomer solution in 50% isopropanol, 25% methanol and 20% water, 1-methoxy, 2-propanol and platinum-C-catalyst (20% by weight platinum on Vulcan black). The ink is then printed on the polymeric membrane with the use of a screen printing procedure. Electrode coating and the membrane are subsequently pressed together at 127.degree. C. under pressure of 20.7 bar. In the description of this patent application, it is mentioned that the electrode coating should be porous. An average pore diameter of 0.01 to 50 microns, preferably 0.1 to 30 microns and porosity of 10 to 99%, preferably 10 to 60% is described. However, for the person skilled in the art it is clear these porosities can not be realistically achieved with the described preparation procedures. This is equally true for the pore diameter as well as for the porosity.
The performance data of fuel cells is highly dependent on the oxidizing agent selected. Maximum values are obtained when pure oxygen is used. When air is used the performance data distinctly drops.
Polymer electrolyte fuel cells, which constitute the subject matter of this invention, are intended to be primarily used as current suppliers in vehicles. Therefore, the goal is to operate the fuel cells with air. The creation of successful gas diffusion electrodes for operation in air is therefore of decisive importance for the successful use of fuel cells as the energy source in motor vehicles.
An object of the present invention is to improve gas diffusion electrodes for membrane fuel cells in respect of performance data during operation in air.
A further object of the present invention is to improve operation of gas diffusion electrodes by controlling the interplay of the three phases of catalyst/electrolyte/gas.
A still further object of the present invention is to create a method of producing these improved gas diffusion electrodes.